Antigenic variation was observed in the expression of specific tumor associated antigens within individual human mammary tumor masses using monoclonal antibodies. This variation was demonstrated by both the pattern and cellular localization of reactivity with a given antibody. This diversity was also observed in human mammary tumor cell lines grown in vivo and in vitro. Analyses of DNA content and cell surface binding of monoclonal antibodies during logarithmic growth phase, and at density-dependent arrest, demonstrated that the expression of some tumor associated antigens is related to S-phase of the cell cycle. Membrane expression of the reactive antigens appeared to be stable despite prolonged exposure to antibody. Antigenic drift was observed with continued passage of mammary tumor cell lines; consistent with this finding, the "same" mammary tumor cell line obtained from different sources exhibited distinct antigenic phenotypes. Single-cell clones derived from the MCF-7 mammary tumor cell line exhibited at least four distinct antigenic phenotypes; a change in cell surface phenotype of some of the clones was seen during subsequent passage.